1. Field of the Invention
The present invention relates to drilling subterranean well bores, specifically to improved under balanced method and apparatus for drilling a well bore through or below a permeable formation containing such fluids as oil, gas, and water.
2. Description of the Related Art
Drilling a well bore typically requires circulating a drilling fluid to flush the bore of cuttings produced by action of a drill bit. The drilling fluid may be pumped down the well inside the drill string and, with picked up cuttings, back to the surface through the annulus outside the drill string. In another form, known in the art as reverse circulation, the drilling fluid is pumped through sealed annulus between a casing and the drill string, and drill cuttings are evacuated through the drill string.
Traditional drilling techniques maintain hydrostatic pressure of the drilling fluid in the well bore higher (“overbalanced”) with respect to the formation pore pressure. In this overbalanced situation, materials are added to the drilling fluid to restrict fluid flow into formation by depositing low permeability filter cake on the borehole wall. Overbalanced drilling prevents formation fluid blowouts. But in certain conditions the drilling fluid flows into permeable formation and circulation may be partially or completely lost. Lost circulation is a costly problem. The production formation may be damaged by invading drilling fluid.
Under balanced drilling (UBD) has been introduced to avoid the shortcomings of the overbalanced drilling. Under balanced drilling is a technique wherein the pressure in an open section of the borehole is intentionally maintained below the formation pressure such that formation fluid flows into the well bore while drilling. Typically formation fluid flowing into the well bore is circulated to the surface with a drilling fluid pumped into the well bore.
Drilling fluids in major under balanced drilling techniques comprise gases, hydrocarbon liquids, water, mixtures of gaseous and liquid phases, foams.
Many UBD operations in depleted fields and in developing lower-permeability reservoirs are successful. But drilling long vertical intervals or horizontal sections in highly permeable formations, especially with cavities and fractures, remains a serious problem. Partial and full lost circulation is often encountered in such conditions. Loss control materials (LCM) are used to regain circulation. Sometimes drilling continues without returns. LCM and cuttings pumped into the formation while drilling without returns may plug the best pay zones, defeating, at least partially, the main goal of under balanced drilling.
In prolific formations UBD drilling fluids can transport to the surface tremendous volumes of fluids. If operator is not ready to treat and utilize the produced oil, UBD can't be implemented. Gas is typically flared at rates often exceeding 5 MMcfd. Water disposal may easily become a costly or prohibitive problem.
In UBD operations the well bore returns tend to be unstable in composition, pressure and rate. The returns may comprise a base drilling liquid, an added gas, drill cuttings, oil, natural gas, formation water, surfactants. The more productive is a formation the more unstable is the wellhead flow as a very small change in the drilling fluid circulating pressure leads to dramatic changes in formation fluid influx rate. It is difficult and costly to handle the unstable wellhead stream, especially if formation fluid is natural gas. Unstable wellhead stream requires separating equipment of big volumes and foot prints.
Many problems in UBD techniques arise in periods of pump off for making connections. The downhole pressure during resuming circulation usually exceeds the formation pressure allowing some volume of drilling fluid to enter the formation and damage it. Prolific liquid producers can kill themselves during times of pump off and often circulation cannot be reestablished. Connections in UBD operations usually take substantially more time in comparison with overbalanced drilling because some additional procedures are required, such as bleeding off, at least partially, the drill string, and repressurizing it thereafter. Methods and apparatus for continuous circulation are disclosed in the U.S. Pat. No. 3,559,739 to Hutchison and U.S. Pat. No. 6,412,554 to Allen, et al. The system of these patents comprises an upper and lower chambers sealingly encompassing adjacent parts of two drill pipe joints to be connected or disconnected; a gate apparatus for temporarily separating the chambers, ports in the chambers in operational connection with bypassing and bleeding lines. This system is designed to be used with direct circulation of a drilling fluid. It cannot be used reliably with a system evacuating drill cuttings to the surface through the drill string, as it takes place in reverse circulation, because cuttings may damage thread of the box. The least erosion of the threaded end of the drill pipe joint may result in severe complications due to washouts.
Deficiencies of UBD under previous art may be characterized by following:                “ . . . more than 15,000 under balanced wells have been drilled on land in the US and Canada as of September 2002, of which only 9,000 were drilled under balanced over the entire planned length and into completion” (Nina M. Rach, “Underbalanced, near balanced drilling are possible offshore”, Oil & Gas Journal, week of Dec. 1, 2003 pp. 39-44).It means that resources invested in UBD drilling of more than 6,000 wells were substantially lost.        
There is a sound need in the industry to overcome above mentioned and some others drawbacks of existing UBD techniques.                3. Objects and Advantages        
The first object of this invention is an UBD method for drilling a well bore through and below a permeable formation wherein the well bore is flushed with the formation fluid being produced, whereby loss circulation problem may be eliminated and loss of production due formation damage avoided
The second object of the invention is the UBD method wherein a flow rate of the formation fluid being produced while drilling may be kept substantially stable, whereby conditions for a cost effective way for handling a wellhead stream and utilizing produced formation fluids may be created
The third object of the invention is the UBD method which may comprise returning any part of produced formation fluid into the well bore being drilled, whereby the problem of disposing the formation fluid may be solved
The fourth object of the invention is a cost effective method for adding to or removing from a drill string a drill pipe joint without interrupting the well bore flushing wherewith the drilling method of the invention is supported.
The fifth object of the invention is a lifting gas injector which allows simultaneously pumping in the same channel a lifting gas and produced formation liquid while practicing the UBD method of present invention, whereby the third objective of the invention is supported by apparatus.
Further objects and advantages will become apparent from a consideration of the ensuing description and drawings